Photographic exposure control circuit



Jan. 19, 1954 B, L vmE 2,666,858

PHOTOGRAPHIC EXPOSURE CONTROL CIRCUIT Filed June 6, 1951 IN VEN TOR.

Bernard Levine BY ATTORNEY Patented Jan. 19, 1954 orsics PHOTOGRAPHIC EXPOSURE CONTROL CIRCUIT Bernard Levine, Bronx, N. Y., assignor to The Fotopak Corp, Englewood, N. J.

Application June 6, 1951, Serial No. 230,159

2 Claims. 1

The present invention relates, in general, to photographic apparatus in which the exposure interval is determined by the rate of change in the charge of an energy storage device, such as a capacitor, as controlled by the intensity of the exposure light.

More specifically, the invention relates to a means for providing that the initial charge upon the capacitor, as predetermined by a particular setting of the apparatus, be maintained at the level preset at the start of the exposure operation.

In photographic xposure control circuits, where a predetermined initial charge upon a capacitor is used to control the exposure operation, it has been found that it is highly desirable to have a minimum of five diiierent settings to selectively provide five different initial charges for the capacitor. In addition, it is expedient to operate the apparatus from an A. C. supply source. However, due to fluctuations in the A. C.

line voltage and other transient conditions, the initial charge upon the capacitor is frequently varied from the preset level thereof whereby to adversely affect the exposure operation. Therefore, it is an object of the present invention to avoid the foregoing disadvantage of the prior art,

Another object is to generally provide for a more efficient and effective exposure operation with a resultant saving in time and material.

Other objects and advantages of my invention will appear from the following description when read in connection with the sole figure of the accompanying drawing, which is a schematic diagram of a control circuit embodying my invention.

Referring to the drawing in detail, there is shown a control circuit for a photographic printer provided with a lamp to. It will be understood that said lamp is positioned to illuminate a record or negative to be printed and that said record is imaged on photographic material by means of a suitable lens. A typical arrangement of the lamp, the record and the lens is illustrated and described in Patent No. 2,353,218 to B. Burnham, dated July 11, 1944. As illustrated in said patent, the photographic printer is also provided with a transparent mirror interposed between the lens and the photographic material, and effective to reflect a portion of the light transmitted by the negative to the light sensitive cell l2. Therefore, it will be apparent that the light sensitive cell I2 is activated in accordance with the printing exposure and assumes a resistance corresponding to the transmission of the negative.

The primary P of the transformer T is adapted 2 to be connected to a suitable source of A. C. potential, an on-off switch being indicated at [4. The secondaries SI and $2, the vacuum tube it, the filter capacitors I8-i8 and the choice 26 connected therebetween constitute the usual full wave rectifier power supply, the secondary S35 providing a filament voltage supply. A gas-filled voltage regulator tube 22 has one of its electrodes connected to the B-plus terminal of the power supply through the lead Ll the voltage dropping resistor 24 and the lead L2. The other electrode of the voltage regulator tube is connected to the B-minus terminal of the power supply through the lead L3. A voltage divider network is connected across the power supply in shunt with tube 22. Said voltage divider is constituted by the potentiometer 24 and the resistors 26 and 23 connected to each end thereof, respectively. Resistor 28 has its other end connected to the B-minus terminal of the power supply. The other end of resistor 26 is connected to the stationary contact 30 of a micro-switch 32, provided with an additional stationary contact 3% and a movable contact 36. Contact 35 is normally biased into engagement with contact 353. Contact 36 is connected to the B-plus terminal through leads M3, L5, L6, Ll, resistor 24 and lead L2.

The exposure interval is here determined by the rate of change in the charge of the capacitor 38 which is connected at one plate thereof, through the resistor 60 to the movable contacts of the switches SWI, SW2, SW3, SW 5 and SW5. The resistor 40, as described in the above men tioned patent,'provides for a desired shortening of short exposures to compensate for the afterglow of the lamp Ill. The switches SW! through SW5 permit for the connection of the capacitor 36 to different points of the potentiometer as of the voltage divider and thereby determine the initial predetermined charge applied to the capacitor 38. The other plate of the capacitor 38 is connected to the aforementioned photo-cell l2 which is connected to lead L4.

Capacitor 33, at said other plate, is also connected through lead L1 to the control grid of tube 42. The cathode of tube i2 is connected to lead L3 and the anode of said tube is connected to lead L6 through the resistor id and the capacitor 46 connected in parallel. Said anode is also connected to the control grid of the thyratron tube 48. The cathode of tube 6% is connected through lead L3 to the stationary contact 34 of the micro-switch 32. The anode of tube 46 is connected to one terminal of a relay winding 50, the other terminal of which is connected.

through lead L9 to one side of the A. C. input supply. A filter capacitor 52 is connected in parallel with the relay winding. The relay contacts, indicated at 54, are open in the de-e-nergized condition of coil 5:] and are connected in series between the lamp It and lead Lt, the lamp being also connected to lead L9.

To operate the apparatus, switch M in the A. C. line is closed to provide the B-plus voltage in the power supply. In this condition of the circuit, that is with switch 52 normally closed at contact 38, the thyratron tube ldhasan open cathode circuit so that there is no current flow through the relay coil Ell. will be noted that one side of the coil is connected to the C; line through lead LG but the circuit from the other side of the coil to the A. C. line, which includes leads Li, L5, L5, L l, switch 34, lead L8 and tube 48 is open at switch 32. Therefore the relay con tacts 54 remain open and the exposure lamp ii! is Lin-energized. it will be noted that-the lamp is connected to one side of the A. 0. line through lead L9 and that it connected to the other side of the A. C. line H rough leads Li, L L5, L5, and the relay contac 5 5, when closed.

However, the closing of switch if; results-in the charging of capacitor at, it being noted that the capacitor is connected to the voltage divider through the resistor a l. It will be understood that the various individual potentiometers, which are connected in shunt to constitute the potentiometer 2 of the voltage divider, are each pro-set to tap a predetermined voltage so that any one of these predetermined voltages may be applied to capacitor 325 by the appropriate one of the switches SW .1 through SW5. Therefore, by closing any one of said switches a predetermined initial charge is applied to the capacitor.

1 is essential that the determined charge be maintained at the predetermined level, prior to the start of the exposure, since the exposure operation is directly related to said initial charge. In the said in tial charge was subject to variation due to fluctuations in the A. C. line voltage or for other reasons. However, pursuant to the present invention, the voltage regulator tube 22 maintains said initial charge at the preset level.

The exposure operation initiated by closing the micro-switch 32 at the stationary contact 34. This completes the circuit between the cathode of thyratron tube at and the A. C. line, as previously described, current then flows through the relay coil Said energization of the coil causes the relay contacts 51} to close and thereby completes the circuit through the exposure lamp it to start the exposure. Upon the starting of the exposure, a portion of the light transmitted by the negative is directed by the previously mentioned reflector upon the photocell l2 and the charge on the condenser 38 begins to drain off at a rate determined by the intensity. of the light transmitted by the negative. It will be understood that since the capacitor 33 is in circuit between the control grid and the cathode of tube 42, the initial charge upon said capacitor is sufficient to bias said tube to cut-off. However, when said charge has drained off to the point where the grid of tube [l2 becomes sufficiently positive to render said tube conductive, plate current begins to flow from said tube to develop an IR drop across resistor M which is in circuit between the control grid and the cathode of tube 48. Said voltage drop across resistor 44 biases the tube 48 to cut-off whereupon the coil 5% is (lo-energized to open the relay contacts as and terminate the exposure operation by de-energizing the lamp l0.

While I have described the preferred embodiment of my invention, it will be obvious that it is not limited to the exact form illustrated and that many variations may be used without departing from the scope of the invention, as set forth in the appended claims.

I claim:

1. In a photographic printing apparatus in which the discharge of a capacitor through a predetermined range and under the control of a light sensitive cell determines the exposure period, a capacitor, an electron tube having a grid connected directly to one plate of the capacitor and having at least an anode and a cathode, a voltage rectifier, said anode being in circuit with the positive terminal of said rectifier, said cathode being in circuit with the negative terminal of said rectifier, a voltage divider for said rectifier, the other plate of said capacitor being connected to said voltage divider for charging said capacitor, a light sensitive cell, connected for discharging said capacitor in accordance with the conductance of said cell, the charge on said capacitor initially biasing said tube to out off current flow therethrough, and said tube being rendered conductive upon said discharge of said capacitor to terminate the exposure period and means for maintaining said charge upon said capacitor at a constant level immediately prior to the start of said exposure period, said latter means being voltage regulator means connected across the output of said voltage rectifier means.

2. In a photographic printing apparatus in which the discharge of a capacitor through a predetermined range and under the control of a light sensitive cell determines the exposure period, a capacitor, an electron tube having a grid connected directly to one plate of the capacitor and having at least an anode and a cathode, a voltage rectifier, said anode being in circuit with the positive terminal of said rectifier, said cathode being in circuit with the negative terminal of said rectifier, a voltage divider for said rectifier, the other plate of said capacitor being connected to said voltage divider for charging said capacitor, a light sensitive cell, connected for discharging said capacitor in accordance with the conductance of said cell, the charge on said capacitor initially biasing said tube to cut off current flow therethrough, and said tube being rendered conductiveupon said discharge of said capacitor to terminate the exposure period, and means for maintaining said charge upon said capacitorat a constant level immediately prior to the start of said exposure period, said latter means being voltage regulator means connected across the output of said voltage rectifiermeans, said voltage regulator means being constituted by a gas diode.

BERNARD LEVINE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,158,903 Knobel May 16,1939 2,251,613 Kott Aug. 5, 1941 2,353,218. Burnham etal July 11, 194.4 2,499,996 Kelsey .Mar. '7, 1950 

